Comparison of conventional inserts and an add-on electron MLC for chest wall irradiation of left-sided breast cancer.
BACKGROUND
Collimation of irregularly shaped clinical electron beams is currently based on electron inserts made of low melting point alloys. The present investigation compares a conventional electron applicator with insert and add-on eMLC-based dose distributions in the postoperative chest wall irradiation of left-sided breast cancer.


METHODS
Voxel Monte Carlo++ (VMC++) calculated dose distributions related to electron fields were compared with 10 left-sided breast cancer patients after radical mastectomy. The prescription dose was 50 Gy at a build-up maximum. The same dose was prescribed for the ipsilateral axillary, parasternal and supraclavicular lymph nodes that were treated with photons and calculated with a pencil beam algorithm. The insert beams were shaped with 1.5 cm thick Wood's metal electron inserts in an electron applicator of a Varian 2100 C/D linac. Doses for the eMLC-shaped beams were calculated for an eMLC prototype with 2 cm thick and 5 mm wide steel leaves. The same collimator-to-surface distance (CSD) of 5.8 cm was used for both collimators.


RESULTS
The mean PTV dose was slightly higher for the eMLC plans (50.7 vs 49.5 Gy, p<0.001, respectively). The maximum doses assessed by D5% for the eMLC and insert were 60.9 and 59.1 Gy (p<0.001). The difference was due to the slightly higher doses near the field edges for the eMLC. The left lung V20 volumes were 34.5% and 34.0% (p<0.001). There was only a marginal difference in heart doses.


CONCLUSIONS
Despite a slight increase of maximum dose in PTV the add-on electron MLC for chest wall irradiation results in practically no differences in dose distributions compared with the present insert-based collimation.
Vatanen
Tero
T
Department of Oncology, Kuopio University Hospital, Kuopio, Finland. tvatanen@hytti.uku.fi
Traneus
Erik
E
Lahtinen
Tapani
T
D001943:Breast Neoplasms / Q000532:radiotherapy* / Q000601:surgery; D003198:Computer Simulation; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006801:Humans; D008168:Lung / Q000528:radiation effects; D008408:Mastectomy; D009010:Monte Carlo Method; D011874:Radiometry; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D020266:Radiotherapy, Conformal / Q000295:instrumentation*; D035441:Thoracic Wall / Q000528:radiation effects*